Modeling of enhanced biological phosphorus removal in a sequencing batch biofilm reactor

~

Pergamon

Wal. Sci. Ttch. Vol. 37, No. 4-5. pp. 583-587. 1998. \l:) 19981AWQ. Pubhshed by ElsevIer Science Lid

Pnnled in Greal Bnlain. 0273-1223/98 S19'00 + 0'00

PH: S0273-1223(98)00169-3

MODELING OF ENHANCED BIOLOGICAL PHOSPHORUS REMOVAL IN A SEQUENCING BATCH BIOFILM REACTOR Eberhard Morgenroth and Peter A. Wilderer Institute of Water Quality Control and Waste Management. Technical University of Munich. D·85748 Garching. Germany

ABSTRAcr A biofilm system operaled for enhanced biological phosphorus removal is evaluated using a mathematical model, The influence of the influent COD concentration and the biofilm thickness are investigated. In an activated sludge system increasing the influent COD will result in a decrease of the effluent phosphorus concentration. However. in a biofilm system above a certain influent COD concentration not all COD supplied in the influent can be taken up during the anaerobic period. Other heterotrophic bacteria will then dominate the biofilm resulting in an increase of the effluent phosphorus concentration. A larger biofilm thickness will result in an increase of the total mass of polyphosphate.accumulating organisms in the system. However. it is shown that a larger biofilm thickness results in higher effluent phosphorus concentrations. The mathematical model presented is based on the IAWQ Model No.2 modified for the biofilm system. Mass transport in the biofilm is modeled one-dimensionally. Removal of biomass through backwashing and. thUS. removal of phosphorus. is included in the mathematical model. Simulations were used to explain experimental observations. 1998 IAWQ, Published by Elsevier Science Ltd

KEYWORDS Enhanced biological phosphorus removal; sequencing balch mathematical modeling; biofilm; IAWQ activated sludge model No, 2.

biofilm

reactor

(SBBR);

INTRODUcrION Biofilm technologies can be advantageous for enhanced biological phosphorus removal (EBPR) because they can help to overcome the following problems encountered in activated sludge systems operated for EBPR: (I) bulking and foaming sludges (Kunst and Reins, 1994); (2) secondary release of phosphorus in Ihe final clarifier; and (3) space constraints that make the construction of activated sludge plants difficult. In biofilm systems the biomass is fixed on a support material and sedimentation is not required for biomass retention, Thus, problems with bulking and foaming and secondary release of phosphorus in a clarifier are avoided. Generally, biofilm systems can be built more compact than activated sludge systems. To achieve EBPR in a biofilm system the biomass needs to be exposed to changing environmental conditions. This can be accomplished using, e,g., a sequencing batch biofilm reactor ( Gonzalez and Wilderer, 1991). Other systems have been applied such as a continuous flow biofilter system w~ere the flow direction and aeration S83

584

E. MORGENROTH and P. A WILDERER

are sequentially changed (Goncalves and Rogalla. 1992). The objective of this paper is to present a mathematical model for EBPR in biofilms and to evaluate different operational strategies using this model.

influent Figure I. Sequencmg batch biofilm reactor (SBBR).

METHODS

The sequencing batch biofilm reactor (SBBR) is a type of fill and draw reactor where the biomass is fixed on a support medium (Figure I). In the simulations presented below the SBBR is operated with an 8 h cycle. The first 2 h of the cycle are anaerobic, followed by 5 h of aeration and I h of draw and idle. The reactor is filled during the first 30 min of the cycle. During the anaerobic or aerobic react periods mixing is provided by recirculation of wastewater. During draw 100 % of the water is drained from the reactor. The volume of water in the reactor is 161. The packing material has a total surface area of 8 m 2. Excess biomass and phosphorus are removed from the SBBR by backwashing once a day. Mathematical model A metabolic model for EBPR in the biofilm is presented in Table I. The proposed model was developed based on the activated sludge model No.2 (Henze et al., 1995). Some modifications were necessary to enable simulation of EBPR in the biofilm: Maintenance. Bacteria require energy to maintain a number of processes that are not associated with growth: maintenance of gradients and electrical potential, support of futile cycles. and turnover of internal macro• molecules (Nielsen and Villadsen, 1994). In the absence of any external substrate bacteria use internal storage products to generate maintenance energy. In mathematical models different approaches can be used to introduce the process of maintenance. In Henze et al. (1995) the effect of maintenance is modeled using the "death-regeneration" approach. A portion of the biomass is considered to lyse ("death"). Subsequently, a fraction of the lysis products is reused by the cells for growth and respiration ("regeneration"). Thus. the net effect of "death-regeneration" is the decrease of active biomass, the utilization of electron acceptors plus a production of nonbiodegradable solids. However, the concept of "death-regeneration" should not be applied in a biofilm system to model maintenance. In the case where no electron acceptor is available in deeper regions of the biofilm, substrate released through "death" would diffuse to other regions of the biofilm and would be used by other organisms for "regeneration". This transfer of internally stored substrate in the respiration of the biomass under aerobic biofilm is unlikely. In the proposed metabolic model (Table conditions is modeled without the release of substrate (process B or D). The production of nonbiodegradable particulate and soluble products is modeled as an inactivation independent of an electron acceptor (process

n.

A,C).

Modeling of enhanced biological phosphorus removal

S8S

Table I. Simplified EBPR Model for the biofilm (numbers and parameters refer to processes and components in Henze et al. (1995), A-D are new processes introduced to model maintenance in the biofilm). Component, i .....

.!. Process,j

Process Rate, PI

Heterotrophic Organisms

_.!.:.:!u.

S Aerobic growth of XH

YH

I'H

A Inactivation OfXH

vA-P04

-I

B Respiration ofXH

1

-iPBM

t YH

fSlI

fxu

SA r~IPo.]x KA +SA On t.imil H

bu.,' XH

-I

-I

i pBM

[

bH.r• p[:]xH

Phosphorus-Accumulating Organisms (PAOs)

-I

10 Storage of XPHA 11 Storage of Xpp 12 Aerobic growth of XPAO

Y P04

-YP04

'YPHA

-I

I

_!=!Ii.

-ipBM

I

YH

C Inactivation o( XPAO

vc.P04

D Respiration o(XPAO

·1

ipBM

14 Lysis o( Xpp

(Xl

PIO -

P, I

bPAO.....[ :]XPAO

(x" IX PAo ) Kpp +(X pp IX'AO)

I

-I

f'AO

txP!!A IXPAO) KMAlC-(Xpp/XPAO) lAir]~xPAo KpHA+(XPHA/XPAO) KIPP+KMAX (Xpp I XpAO ) On Kps+SP04

[(XP!!A IXPAO) IAirlPO.lv 'PAO KPHA+(XPHA'XPAO) 011 UmilfPAO

[~]- Ko:~~o"

[::]- K:'r;~ro.

bpp,,' XPP.J

·1

1

q'""[~I KA+SA

PII - qpp[

brAO.1 • XPAo

-1

vAJ'Oot - vc~ - ipBM • (S1.riP$,l - fXl.J·iPXJ

P,G

-YPHA PII -INH PI'

-I

1

IS Lysis o(XPHA where:

fSlI

I

bu.I

-

bmA.J . XmA.1 (I - (I • (s)'(1 - (Xl»-tIH

bu.r.." - (I • YH)-(I • (5,)-(1 - fXl)-bH

brAOJ - (I • (I • fSl)-(1 • fX!l)'brAQ brAO..... - (l - YHl'(I • fSl)'(I - fX!l'brAO b".1 - (l - (I - fSl)-(1 - fX!l)'b pp brHAJ - (I - (I - fsl}'(I - (Xl»'b PHA fXl.J - (I • (I • fSl)-(1 - fXl»" 'fXl fSIJ - 1 - f XlI

Backwashin~. The only sink for phosphorus in the system is the removal of phosphorus-rich biomass through backwashing. In the presented model the process of backwashing is included by removing all biomass above a defined base thickness once a day at the end of an aerobic period. Approaches used in other models to control the biofilm thickness do not include backwashing and cannot be applied to describe EBPR: Wanner et al. (1994) assume a constant detachment velocity. Wanner and Reichert (1996) assume a constant biofilm thickness with the detachment of all biofilm that exceeds the given biofilm thickness.

Simplifications. To increase computational speed not all processes and components from Henze et al.(I995) are included in the presented model (Table I). Processes related to nitrogen removal are not considered, all organic substrate is assumed to be in the form of acetate and. thus, hydrolysis and fermentation are not considered. Simulation. Mass transport limitation and organism distribution in the biofilm are based on a one• dimensional biofilm model. For the simulation of the biofilm reactor the computer program AQUASIM was used (Wanner and Reichert, 1996). All simulations were run until a steady biomass distribution was achieved in the biofilm which required the simulation of tOO days. The simulation is very time consuming (40 h for a 100-day simulation on a Pentium PC) because discrete changes such as fill and draw. changing environmental conditions, and backwashing slow down the numerical algorithm.

586

E. MORGENROTH and P. A WILDERER

RESULTS AND DISCUSSION Calculations with the model presented above were performed using the kinetic and stoichiometric default values proposed by Hellze et al. (\995) and the parameters defined in Table \. The standard wastewater in the simulation consisted of acetate (SA = 200 mg \-1) and phosphate (SP04 20 mg \-1). The biofilm base thickness was 500 ~m (= biofilm thickness after backwashing). The effect of varying the influent SA concentration is discussed in Figure 2; the effect of varying the biofilm thickness after backwashing is discussed in Figure 3.

=

Influent acetate concentration.' If the influent acetate concentration is increased in an activated sludge system operated at a constant sludge age the effluent phosphorus concentration will decrease. However, in the biofilm system increasing the influent acetate concentration will improve the effluent quality only up to a certain limit (Figure 2). If the influent acetate concentration is increased further from 400 to 600 mg I-I the effluent phosphorus concentration increases to \7 mg I-I. It can also be observed that by increasing the influent acetate concentration (up to 400 mg I-I) the phosphorus release at the end of the anaerobic period increases above 100 mgP I-I. However, if the influent acetate concentration exceeds 600 mg I-I no phosphorus is relea~ed during the anaerobic period (b). The reason is that the surface area of the support media limits the active biomass concentration. At high COD loadings not all organic substrate can be removed from the bulk liquid during the anaerobic phase. During the following aerobic phase other heterotrophic organisms use the bulk liquid COD and become the dominant species in the biofilm resulting in the loss of EBPR. The optimal COD surface loading of 0.\ g m- 2 h-I (or SA,influent = 400 mg 1. 1) from the simulation compares well with the experimental findings of Garzon-Zuniga and Gonzalez (1996) of 0.\25 g

m- 2 h- l .

25 0:::

Clll

~

IS

~

10

0:::

Clll

E

~

VJ

.

(b) S& A

onocrobic .......ic

100

20

E

E

120

a)

VJ

5

50 200 400 600

80

60 40 20

0

0 0

250

500

7SO

1000 12SO

0

2

4

6

8

cycle time, h mfluent SA' mgll Figure 2. Influence of the acetate concentration in the influent on
Bia!ihn thickness: In Figure 3(a) the distribution of the polyphosphate-accumulating organisms (XpAO> over the height of the biofilm is shown for two different biofilm thicknesses (base thickness after backwashing = 500 or 5000 ~m). The thicker biofilm contains twice the mass of X pAO whereas in the thinner biofilm the concentration of X pAO is higher at the biofilm surface. Phosphorus release and subsequent uptake is higher in the thinner biofilm (Figure 3b) resulting in a lower effluent concentration. The reason for the better performance of the thinner biofilm is that stored polyphosphate is removed from the system only in that part of the biofilm that grows above the base thickness. Thus, even though the total mass of X pAO is larger for the thick biofilm the mass of X pAO and also the mass of stored polyphosphate grown above the base thickness is larger for the thin biofilm. The simulated results compare well with experimental results. Gonzalez and Wilderer (1991) reported a very thick biofilm in the start-up phase without achieving EBPR. Effective EBPR could only be established after most of the biofilm had sloughed off and a thin and stable biofilm had developed. For practical applications it is important to note that deeper regions of the thick biofilm contain only little active biomass (Figure 3a). Thus, if the thick biofilm sloughs off it will take considerable time until enough active biomass is available again to achieve effective EBPR. A thin biofilm consists of active biomass throughout the depth of the biofilm and in addition the thin biofilm is not as susceptible to sloughing as thick biofilms.

Modeling of enhanced biological phosphorus removal

1,0

;/ ...~ X

100

(a)

0,8

_ i c ocrobic

587

(b)

80

SOO m

0,6 0,4

~

Ei

60

~

40

CIl

0,2

20

0,0

0 0,0

0,5 relative biofilm thickness

1,0

0

2

4 6 cycle time, h

8

Figure 3. Influence of the biofilm thickness (SOO and 5000 j.lm) on (a) the fraction of the polyphosphate accumulating organisms (XpAO) in the biofilm and (b) the bulk phase concentrations during a cycle. (Simulations with SP04.•nnuent" 20 mgP I" and SA...nuent 200 mgCOD 1. 1)

=

CONCLUSIONS A mathematical model for EBPR in the biofilm was presented and was used to explain experimental observations. If the influent COD is in an appropriate range effective EBPR can be achieved in a biofilm system.

Thin biofilms are advantageous for EBPR. Even though a larger biofilm thickness will increase the total mass of phosphorus-accumulating organisms the effluent phosphorus concentration will increase. In future work the kinetic, stoichiometric and biofilm parameters need to be calibrated for the system. The metabolic models developed in activated sludge systems need to be verified for EBPR in biofilms. ACKNOWLEDGEMENT The research was funded by Oswald-Schulze-Stiftung (Germany) grant no. AZ 904/95 REFERENCES Garzon·Zuniga, M. A. and Gonzalez-Martinez, S. (1996). Biological phosphate and nitrogen removal in a biofilm sequencing batch reactor. Wat. Sci. Tuh. 34(1-2), 293-30l. Goncalves, R. F. and Rogalla, F. (1992). Continuous biological phosphorus removal in a biofilm reactor. Wat. Sci. Tuh. 26(9-11), 2027-2030. Gonzalez·Martinez, S. and Wilderer, P. A. (1991). Phosphate removal in a biofilm reactor. Wat. Sci. Tuh. 23(7-9),1405-1415. Henze, M.. Gujcr, W., Mino, T., Matsuo, T., Wentzel, M. C. and Marais, G. v. R. (1995). Activaled sludge model No.2. IAWQ Scientific and Technical Reports, No.3, IA WQ, London. Kunst, S. and Reins, M. (1994). Practical investigations on bulking and foaming in activated·sludge plants with biological phosphorus removal. Wat. Sci. Tuh. 29(7), 289·294. Nielsen, J. and Villadsen, J. (1994). Bioreaction Engineering Principles, Plenum Press, ISBN: 0-306-44688.X. Wanner, 0., Debus, O. and Reichert, P. (1994). Modeling the spaliaJ-distribulion and dynamics of a ltylene·degrading microbial . population in a membrane·bound biofilm. Wat. Sci. Tech. 29(10-11),243·251. Wanner, O. and Reichert, P. (1996): Malhematical-modeling of milted-cullure biofilms. Biolechnol. Bioeng. 49(2), 172-184.